Don't believe the critics and test it... it's really fun and worth it!
I've completed it as Human and Na'vi and would give it a final score of around 75%.
The graphics are just amazing and the jungle is so breath-takingly beautiful that you'll stop from time to time to just absorb all the beauty! I'm a huge fan of jungle-like environments in general and I particularly loved the jungle in the Avatar movie. The many trees, flowers, and grass in this game give you pretty much the same feeling as in the movie... they're just everywhere :) ! The jungle was so great in fact that I never took a transport (e.g. buggy, horse etc.) and decided to walk instead ;) !
The animations are very impressive too! Whether you hit a Human with your powerful staff as a Na'vi or whether you shoot a Na'vi with one of your many weapons, everything looks great in my opinion.
The sound effects are good too and really give you the feeling of walking through a living jungle, i.e. you might hear birds or insects in the background while wandering through the bush.
The music isn't as great as in the movie but good enough to keep you entertained. Whenever you enter combat it changes to a more dramatic tune to adept to the current situation.
The controls are supposed to be the worst ever created. Well, this could actually be true for some vehicles but never for the entire game! If you play your RDA-Trooper or the Na'vi soldier only, so without using any vehicles whatsoever, you won't notice anything different when it comes to the controls. WASD is moving your character, the mouse is used to look around, left mouse-button shoots and the right mouse-button can be assigned to whatever you want (I used dodge) and the spacebar is jump. Sounds familiar? Exactly! This is the standard control scheme for pretty much every action game out there... so you'll love it! Some vehicles (particularly the Banshee from the Na'vi or all buggies from the RDA) are a pain in the backend to control though! But again, if you love the jungle as much as I did you will almost never ride one of those anyway. There are some scenes where you have to ride the Banshee for instance but fortunately they're short and you don't really have to worry about actually doing something except flying from A to B.
One feature I was really missing though is a difficulty setting. The game is just too easy and you hardly ever die. In my opinion being a Na'vi is a bit more difficult than being a RDA-Trooper but both campaigns are pretty easy to complete and will take you about 10 hours each.
This is actually another nice feature in Avatar: after about one hour into the game you have to decide whether you wanna play as a Na'vi or a Human. Basically you could say that you have two games in one. Still, it wouldn't have hurt to include the possibility to adjust the difficulty of the game. I guess a work-around for this would be to play the game with the very basic gear and weapons instead of using your many upgrades throughout the game.
Because for everything you kill and for every quest you complete you're gaining experience. After a certain amount of experience points you can unlock new weapons, better armor and more effective and new skills. Those ones are actually really cool but make the game even easier so you might wanna use the very basic stuff in order to keep the game difficult.
The story isn't really that great and original but I had this with a couple of other previous games too. With Avatar it's a bit more a shame though since the movie was really great in my opinion and they could have made the game so much better. Even though the game plays as a prequel to the movie they don't really feed you with too many information and details about the planet, the characters and your mission in general.
All in all though this game was so much fun to play and I strongly suggest giving it a try, no matter what all the other critics say! There's a free demo available to download if you wanna have a free look into it before actually buying the game.
I'm definitely gonna play it again because the action is good and entertaining and the graphics are just amazing!